Isotactic polypropylene can be produced by the polymerization of propylene in the presence of catalysts such as Ziegler Natta catalysts or isospecific metallocene catalysts. Isotactic polypropylene can be used in the production of molded articles in which the polypropylene is heated and then extruded through one or more dies or nozzles into a mold cavity in which it moves in both a longitudinal direction (referred to as the flow direction) and in a transverse or lateral direction (sometimes referred to as the cross flow direction). The structure of isotactic polypropylene is characterized in terms of the methyl group attached to the tertiary carbon atoms of the successive propylene monomer units lying on the same side of the main chain of the polymer. That is, the methyl groups are characterized as being all above or below the polymer chain. Isotactic polypropylene can be illustrated by the following chemical formula:
Stereoregular polymers, such as isotactic and syndiotactic polypropylene, can be characterized in terms of the Fisher projection formula. Using the Fisher projection formula, the stereochemical sequence of isotactic polypropylene as shown by Formula (1) is described as follows:
Another way of describing the structure is through the use of NMR. Bovey's NMR nomenclature for an isotactic pentad is . . . mmmmm . . . with each “m” representing a “meso” dyad, or successive methyl groups on the same side of the plane of the polymer chain. As is known in the art, any deviation or inversion in the structure of the chain lowers the degree of isotacticity and crystallinity of the polymer. In the case of random ethylene propylene copolymers, a relatively low ethylene content in the copolymer is randomly distributed throughout the polymer chain so that ethylene units are randomly interposed between the repeating propylene units.
Polypropylene can be formed into molded articles through various operations in which the polymer is heated and conformed to a desired shape and then cooled at to arrive at the final product. Two well known operations involve injection molding and thermoforming.
In injection molding operations, the molten polymer is introduced into a mold cavity. The molten polymer is retained in the cavity for a sufficient time to allow the desired component to form. The period of time required for cooling and subsequent removal of the molded component from the mold cavity is an important factor in the manufacturing efficiency of the injection molding operation.
In thermoforming operations the molten polymer is subjected to a sheet extrusion operation followed by thermoforming of the sheet over a template to arrive at the desired shape of the thermoformed article followed by cooling of the article which is then recovered from the template. Typical thermoforming operations may be carried out employing sheet extrusion and formation of a sheet roll which is then used in a roll fed continuous thermoforming unit. Thermoforming may also be carried out with integrated in-line systems in which a heat extrusion system and thermoforming system are integrated into one automated unit. After the thermoforming operation, the thermoformed article is cooled and then trimmed as necessary to arrive at the final product followed by recovery of the product from the automatic system.
During the manufacturing of formed plastic components by injection molding, shrinkage within the mold and subsequent withdrawal of the hard component from the mold results in a volume difference between the initial and the final molded article. If the dimensional changes are relatively uniform in the transverse (cross flow) and longitudinal (flow) directions of the mold, the shrinkage characteristic is considered to be isotropic. With significant differential dimensional changes in the transverse and longitudinal directions, the dimensional changes are characterized as anisotropic or differential. Warpage is caused by variations in shrinkage throughout injection molded part (D Rosata, Injection Molding Handbook, Chapman & Hall, New York, 1995). More anisotropic shrinkage often leads to warpage problems in injection molding applications. Regardless of whether the shrinkage characterized is isotropic or anisotropic, the relative shrinkage should be taken into account in order to obtain the end use articles of the molded article of the correct dimension. Similar considerations can apply in the case of sheet extrusion and thermoforming operations. In this case, the direction of the continuous linear extrusion leading from the sheet extrusion system to the thermoforming system can be considered to be a longitudinal flow direction and the transverse direction across the direction of the flow of the sheet extrusion system can be considered to be the transverse direction.